This invention relates to indicators for signalling the occurrence of internal faults in oil-filled or gas-filled electrical equipment, such as transformers, reactors, capacitors and the like. The invention has particular application to electrical components used in electrical power distribution systems. In particular, the invention relates to internal fault indicators which display a visible indicator when an abnormally fast increase in pressure is detected within the housing of an electrical device.
Electrical power distribution grids use electrical components, such as transformers, capacitors, and reactors. Potentially dangerous conditions can be created in such devices when aging or operating stresses cause the insulation system to fail. A short circuit within such a device can release a large amount of energy within a fraction of a second. In the worst case the device can explode due to rapid internal pressure buildup from the vaporization of insulating oil and the decomposition of the oil vapor into combustible or volatile gases.
Nearly all pole mounted distribution transformers are protected by a cutout which includes an expulsion fuse or some other fast acting protective device. Such cutouts can minimize damage by disconnecting a faulty device from its source of electric energy so as to interrupt arc current in the event of an overload or internal fault. Service personnel can also use cutouts as manual switches for energizing or disconnecting particular circuits. If there is an overload in the system and the cutout operates, then service personnel can easily spot the open cutout and know that the transformer disconnected by the open cutout is out of service. If the fault is downstream of the transformer then, once that fault has been corrected, it is a simple matter for service personnel to re-fuse the cutout to re-energize the circuit.
If the fault is in the transformer then closing the cutout before the transformer has been repaired will likely produce arcing within the transformer. A device that has failed once is certain to fail again if it is re-energized before the internal damage caused by the arcing has been corrected. Arcing can leave carbonized paths within the device and may impair the mechanical integrity of the device""s housing, or xe2x80x9ctankxe2x80x9d. This increases the risk that the device will fail catastrophically if it is re-energized. In extreme cases the transformer may explode. This could cause property damage and serious injury to service personnel and any members of the public who happen to be close by. To avoid this possibility service personnel must perform careful inspections and take special precautionary measures before attempting to re-energize any electrical apparatus found disconnected from the power system by its protective device.
Unfortunately, an internal fault can occur in a device without leaving any obvious visible cues that the fault has occurred in that device. Unless service personnel can tell that a particular device has failed they may reapply power to the device without detecting that the electrical device has failed. This may cause catastrophic failure of the electrical device, as noted above.
It is known that there is a transient surge in pressure inside oil-filled electrical devices, such as transformers, when the devices suffer from an internal arcing fault. This happens because arcing produces a marked increase in temperature which vaporizes some of the oil. Some electrical devices are filled with electrically insulating gases such as SF6. In such gas-filled devices arcing causes pressure surges in the gas.
There exist fault detectors capable of providing a visual indication that a device has failed. Such fault detectors accelerate the restoration of services while minimizing the possibility that a failed device will be re energized as a result of a human error. U.S. Pat. No. 5,078,078, invented by Cuk, who is also the inventor of this invention, describes a device for detecting transient surges in pressure within the housing of a transformer or similar device. The device fits in an opening in a casing of the transformer. A moveable piston senses rapid pressure surges which result from internal arcing faults within the transformer. The piston has at least one aperture in it so that slow increases and decreases of pressure within the transformer do not cause significant motion of the piston. An indicator attached to the piston changes appearance when the piston has moved a predetermined distance and retains the indication until reset. A disadvantage of the Cuk device is that the change in appearance of the indicator may not be readily apparent, especially from a distance. Furthermore, there is no easy way to prevent false triggering during transport and installation.
U.S. Pat. No. 5,623,891 discloses another device for detecting transient surges in pressure within the housing of a transformer. The device has a diaphragm which is subjected to internal pressure surges within the transformer. The diaphragm carries a trigger retainer which engages a trigger on an indicator shaft mounted for rotation within the housing. An indicator is mounted on the indicator shaft beneath a lens which is visible from the outside of the housing. A bias spring biases the indicator shaft toward rotation relative to the housing when the trigger is engaged with the trigger retainer. When the diaphragm moves in response to a pressure surge in the transformer, the trigger retainer moves away to release the trigger, and the bias spring rotates the indicator shaft and indicator so that a warning section on the indicator is positioned for viewing through the lens.
The device disclosed in U.S. Pat. No. 5,623,891 has a number of disadvantages including:
The inner surface of the indicator lens can become fouled by fumes generated during an internal fault;
The outer surface of the indicator lens can be obscured by ice or snow;
The device provides no way to prevent false triggering during transport and installation;
It is typically necessary to mount the device o n the top surface of a transformer, in a position where the lens is not readily visible from the ground;
The device may be falsely triggered by oil splashing or the like; and,
If the device is to be mounted on the side of a transformer housing then there must be a larger than usual air space in the transformer housing or the diaphragm will be partially submerged.
There is a need for internal fault indicators for electrical equipment of the type used in electrical power distribution which provides a clear visual indication that a device has experienced a fault and which avoids some of the disadvantages of the prior art.
This invention provides a warning indicator for oil-filled electrical equipment, such as power transformers or the like. The warning indicator detects pressure surges created in the housing of the electrical device by an internal electrical fault and yet is insensitive to pressure changes due to normal temperature variations. The indication may be used to positively identify an electrical device which has suffered from an internal fault so that service personnel will be warned not to undergo potentially hazardous attempts to re-energize a faulty piece of electrical equipment.
Accordingly, one aspect of the invention provides a fault indicator for indicating the occurrence of a rapid pressure surge within a housing of an electrical device. The fault indicator comprises: a barrel capable of being mounted in an opening in a housing of an electrical device; and an actuating mechanism. The actuating mechanism comprises: a chamber within the housing, the chamber having at least one orifice communicating between interior and exterior surfaces of the chamber; and, an actuating member movable in responses to a pressure differential between the interior and exterior surfaces of the chamber. The fault indicator also has a plunger within a bore of the barrel. The plunger is biassed outwardly in the barrel and is normally retained in an armed position by the actuating member. When the pressure differential is positive, the actuating member is moved and thereby permits the plunger to move outwardly in the bore to a triggered position.
Preferably the chamber comprises a diaphragm and the actuating member is attached to the diaphragm. The actuating member preferably comprises a trigger pin which projects from the diaphragm and engages a trigger notch in the plunger when the plunger is in its armed position. The diaphragm and the barrel are both preferably oriented generally horizontally. This makes the fault indicator compact.
Another aspect of the invention provides a fault indicator for indicating the occurrence of a rapid pressure surge within a housing of an electrical device. The fault indicator comprises: pressure surge detecting means for moving an actuating member in response to a rapid rise in pressure within a housing of an electrical device; indicator means actuated by the pressure surge detecting means, the indicator means comprising a plunger movably disposed within a bore, the plunger movable outwardly in the bore from an armed position to a triggered position upon movement of the actuating member; and, retaining means for preventing the plunger from becoming separated from the fault indicator.
A still further aspect of the invention provides a method for indicating the occurrence of a rapid pressure surge within a housing of an electrical device. The method comprises: providing a chamber within a housing of an electrical device, the chamber comprising an enclosed volume and an orifice communicating between the enclosed volume and an air space within the housing and providing a plunger having a hidden portion which is hidden from view; allowing a rapid pressure surge within the housing to displace a wall portion of the chamber inwardly; in response to motion of the wall portion releasing a plunger; and, moving the plunger so that the portion of the plunger which was hidden from view is exposed.
Further features and advantages of the invention are described below.